1. Field of the Invention
The present invention relates to an improved pulping process to increase pulp yield, and more particularly it relates to the utilization of wood particles having a maximum thickness of 2 mm and polysulfide in the cooking liquor.
2. Description of the Prior Art
In kraft pulping operations, where the goal is to remove lignin while retaining carbohydrates, yield is increased by minimizing carbohydrate (i.e., cellulose and hemicellulose) degradation. Degradation occurs through a "peeling" reaction in which sugar units are sequentially removed from the reducing end group of the polysaccharide chains. One way to prevent this reaction is to convert aldehyde groups on the wood polysaccharides to a form which is relatively inert to further "peeling." This conversion is achieved by either oxidizing the aldehyde to its corresponding carboxylic acid Alfredsson, B., Samuelson, O. and Sandstig, B. Carboxyl end groups in sulfate and polysulfide pulps. Svensk Papperstidn. 66(18):703 (1963) and U.S. Pat. No. 4,012,280, Holton) or, alternatively, reducing it to its alcohol form (Hartler, N. Sulphate cooking with the addition of reducing agents. Part 1. Preliminary report on the addition of sodium borohydride. Svensk Papperstidn. 62(13):467 (1959) and Pettersson, S. E. and Rydholm, S. A. Hemicelluloses and paper properties of birch pulps. Part 3. Svensk Papperstidn. 64(1):4 (1961)). The two methods that are currently employed involve an oxidation process and use anthraquinone (U.S. Pat. No. 4,012,280), or polysulfide (Clayton, D. W. and Sakai, A. Multi-stage polysulfide pulping processes. Part I). Basic ideas and low-temperature impregnation studies on black spruce heartwood. Pulp Pap. Mag. Can. 68(12):619 (1967); Landmark, P. A., Kleppe, P. J. and Johnsen, K. Cooking liquor oxidation and improved cooking technique in polysulfide pulping. Tappi J. 48(5):56 (1965); Sanyer, N. and Laundrie, J. F. Factors affecting yield increase and fiber quality in polysulfide pulping of loblolly pine, other softwoods, and red oak. Tappi J. 47(10):640 (1964); and Teder, A. Some aspects of the chemistry of polysulfide pulping. Svensk Papperstidn. 72(9):294 (1969)), or both as the oxidizing agents. Anthraquinone is a catalytic additive while polysulfide is generated from white liquor by oxidation of sodium sulfide in one of several processes (U.S. Pat. Nos. 5,082,526, Dorris; and 4,024,229, Smith et al). Of the two yield enhancing agents, anthraquinone is the more stable and can be used at the highest temperatures applied during the pulping process (Fleming, B. I., Kubes, G. J., MacLeod, J. M. and Bolker, H. I. Polarographic analysis of soda-anthraquinone pulping liquor. Tappi J. 62(7):55 (1979)). In contrast, it is known that polysulfide and carbohydrates react at an appreciable rate only at temperatures above 110.degree. C. (Clayton et al., above), while the competing reaction, decomposition of polysulfide to sulfide and thiosulfate, becomes very rapid above 130.degree. C. (Clayton et al., 1967, above and Gustafsson, L. and Teder, A. The thermal decomposition of aqueous polysulfide solutions. Svensk Papperstidn. 72(8):249 (1969)).
The reactions of the yield enhancing agents with the sugar components involve a liquid-solid phase interaction. The reagent ions have to penetrate the wood chips before they can react with the polysaccharides. Anthraquinone, because of its temperature stability, has sufficient time during the progression of cooking to penetrate into the wood chips. In contrast, polysulfide has, to a first approximation, only the time that the temperature remains below 130.degree. C. to penetrate into the wood chips. For this reason, polysulfide effectiveness is generally improved by impregnating wood chips with polysulfide liquor at temperatures below 100.degree. C., and then having a polysulfide treatment period of 15 to 30 minutes, at temperatures between 110 and 130.degree. C. This procedure delays application of the higher cooking temperatures that lead to very fast polysulfide decomposition.
In contrast to conventional kraft cooking, sawdust pulping in M&D (Messing & Durkee) digesters, for example, involves a very rapid rise to temperature, typically less than 5 minutes to 185.degree. C. This time to temperature is too short to provide sufficient time at temperatures between 110 and 130.degree. C. to permit the polysulfide to react effectively and efficiently with the carbohydrates.
This assumption has also governed the operation of conventional systems digesters which incorporate time for impregnation, a slow rise to temperature and chip thickness screening which targets chip accepts, for example, between 2 and 8 mm for softwood kraft pulping.